A tree is defined as a perennialplant consisting of one or more large woody stems. The component parts of a tree are the roots, trunk(s), branches, twigs and leaves. A small group of trees growing together is called a grove[?] or coppice, and a large population of mixed tree species is called a forest.
Tree stems consist mainly of transport tissues (xylem and phloem). In fact, wood consists of xylem cells, and the bark is primarily made of phloem.
The roots of a tree are generally embedded in earth and absorb water and nutrients from the surrounding soil. Above ground, the trunk gives height to the leaf-bearing branches to aid in their competition with other trees for sunlight. In many tree species, the branches spread so as to present the largest possible leaf surface area to the sun.
A tree may not have all the organs mentioned here: most palms do not have branches, the saguaro of North America has no functional leaves, and tree ferns do not have bark. Based on their rough shape and size, all these are nonetheless considered trees.
Some types of tree can grow to over 100 meters tall and/or live for several millennia if circumstances are optimal. A plant that is similar to trees, but generally having smaller, multiple trunks, is often called a shrub, although boundary between the two categories is not precise.
Several biotopes are defined largely by the trees that inhabit them, for example, the rainforest and the taiga.
Trees show a wide variety of leaf types and shapes, bark, flowers, fruit, etc. Trees occur in several diverse families of plants. The earliest trees were probably tree ferns, which once grew in vast forests. Later the conifers, ginkgos and cycads appeared (modern cycads no longer appear as trees). Most species of trees today are flowering plants, which were the most recent to appear. The list below gives some examples of well known trees and how they are typically classified.
